The Concept of Homology in Biology*

In this paper certain long-standing problems in the definition and interpretation of biological homology are discussed. A logical reconstruction of homology is proposed, and shown to generate a computable process for finding homologies. The relations between homology, embryology, evolution theory, and taxonomy are considered briefly. Most recent textbooks define biological homology in terms of common ancestry; e.g. De Beer ([6], p. 478), 'The sole condition which organs must fulfil to be homologous is to be descended from one and the same representative in a common ancestor', and Simpson ([28], p. 78), 'Homology is resemblance due to inheritance from a common ancestry'. Often it is contrasted with analogy, resemblance due to common function. A frequently cited example of homology is the relation between the arm of a man and the wing of a bat, and of analogy the relation between the wing of a bat and the wing of a butterfly. Recently many writers have objected to this kind of definition of homology on the grounds that it is viciously circular, and the objection seems to be well-founded. By these definitions, in order to show that part x in one organism is homologous with part y in another organism, it must be shown that they are derived from a part z in a common ancestor. But homology itself is invoked in identifying parts x and y with part z. The objection is not merely that, since fossil series are incomplete, homology can only be inferred indirectly; however complete the fossil series, these definitions remain invalid, for homology is invoked even in identifying parts of parents with parts of their offspring. How has this muddle: arisen?

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